In a wavelength division multiplexed (WDM) optical network, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or “wavelength channels”, are combined and transmitted through a series of spans of optical fiber. At a receiver end, the wavelength channels are separated and detected by optical receivers.
The optical signals to be encoded are usually provided by laser diodes, one laser diode per one wavelength channel. It is desirable to provide backup laser diodes for redundancy purposes. In view of multiple wavelengths used in a dense WDM (DWDM) transmission, tens and even hundreds of wavelengths in some cases, providing a separate backup laser diode for each wavelength may become prohibitively expensive. Tunable laser sources help solve this problem.
Tunable laser sources also prove valuable in reconfigurable WDM optical networks, in which new wavelength channels are added as a network load increases. Adding and dropping wavelength channels in such a “wavelength-agile” network may be done dynamically, in response to fluctuating data bandwidth requirements between various network nodes. From the network architecture standpoint, it may be preferable to have laser sources tunable to any desired wavelength. Such sources have to be widely tunable, provide sufficient output optical power, and have strong side mode suppression to avoid coherent crosstalk with other wavelength channels.
Referring to FIG. 1A, an exemplary prior-art tunable laser source 100 is shown. A similar laser source is described, for example, in U.S. Pat. No. 5,325,392 by Tohmori et al. The laser source 100 includes optically serially coupled a rear mirror 102, a gain section 104, a phase section 106, and a front mirror 108. The front 108 and rear 102 mirrors include optical gratings having a periodic wavelength dependence of reflectivity. Turning to FIG. 1B, an example wavelength dependence 112 of the rear mirror 102 reflectivity has a period of 5.6 nm. A wavelength dependence 118 of the front mirror 108 reflectivity has a larger period of 6.3 nm. Peaks 112A, 118A of the wavelength dependencies 112 and 118 overlap at 1550 nm. As a result, a product wavelength dependence 130, obtained by multiplying the rear 112 and front 118 wavelength dependences, has its biggest peak 132 at 1550 nm. The product wavelength dependence 130 is shown in FIG. 1B magnified by a factor of four. The product wavelength dependence 130 is proportional to a round trip optical gain for light circulating between the front 108 and rear 102 mirrors of the laser source 100 (FIG. 1A). The product wavelength dependence 130 (FIG. 1B) determines wavelength emission properties of the laser source 100. Three longitudinal resonator modes 121, 122, and 123, denoted with cross (“+”) signs superimposed on the product reflectivity trace 130, are disposed within the 1550 nm peaks 112A, 118A. Additional modes 134, 136, and 138 are present near 1544 nm (134) and 1556 nm (136, 138). Of these modes 121, 122, and 123, 134, 136, and 138, only the central mode 122 results in generation of a laser beam 109 of substantial optical power due to its much higher round trip gain; emission at the side mode 122, 123, 134, 136, and 138 wavelengths occurs at much lower optical power level.
The laser source 100 is tuned by shifting the wavelength dependencies 112 and 118 in opposite directions. When two other peaks of the wavelength dependencies 112 and 118 overlap at another wavelength, lasing occurs at one of longitudinal modes at that wavelength. In essence, the lasing wavelength is tuned using a Vernier effect over wavelength range that is much wider than a wavelength range of tuning the individual mirrors 102, 108 themselves. The wavelength tuning occurs in stepwise fashion. A proper selection of longitudinal mode spacing and reflectivity periods of the back 102 and front 108 mirrors allows one to define a desired magnitude of the wavelength step.
Referring now to FIG. 1C with further reference to FIG. 1A, an exemplary prior-art amplified laser source 150 is shown. A similar laser source is described, for example, in U.S. Pat. No. 6,788,719 by Crowder. The amplified laser source 150 includes the laser source 100 of FIG. 1A and an integrated semiconductor optical amplifier (SOA) 130 serially optically coupled to the front mirror 108. The addition of the SOA 130 allows one to boost the output power of the laser beam 109 to much higher levels than those achievable in the laser source 100 of FIG. 1A. However, the SOA 130 generates additional spontaneous emission noise. Furthermore, the amplification by the SOA 130 is not spectrally uniform across an amplification band due to so-called gain tilt. As a result, the SOA 130 may amplify side modes of the laser beam 109 more than the fundamental mode, reducing side mode suppression ratio (SMSR). For example, the SMSR may be reduced from 50 dB in the laser source 100 to less than 40 dB in the amplified laser source 150 for lasing wavelengths away from the gain spectrum peak. The SMSR degradation may be unacceptable in many applications including a tunable laser source application for a wavelength-agile optical network. A tradeoff exists in the prior art between output optical power and spectral purity of an amplified widely tunable laser source.